A lithographic printing plate ordinarily comprises an oleophilic image area receiving an ink in the printing step and a hydrophilic non-image area receiving a fountain solution. With conventional lithographic printing plates, it is common to mask-expose a PS plate comprising a hydrophilic support having provided thereon an oleophilic photosensitive resin layer through a lith film and to dissolve away the non-image area with a developing solution, thus making a printing plate.
In recent years, a computer electronically processes an image as a digital information, accumulates the digital data, and outputs them. Therefore, it is preferred that, in the image-forming process based on digital image information, image formation be conducted directly on a lithographic printing plate precursor without a lith film by scanning exposure using actinic radiation having a high directivity such as laser light. The technique of making a printing plate from digital information without a lith film is called computer-to-plate (CTP) technique.
In applying the computer-to-plate technique to a process for making a printing plate using a conventional PS plate, there arises a problem that a wavelength region of laser light does not match with a photosensitive wavelength of photosensitive resin.
Also, with the conventional PS plate, a step of dissolving away non-image areas (development processing) after exposure is indispensable. Further, after-processing steps such as a step of washing the development-processed printing plate with water, a step of processing it with a rinsing solution containing a surfactant, and a step of desensitization with a desensitizing solution containing gum Arabic or a starch derivative have also been necessary. It has been a point to be studied with respect to the conventional PS plates that the additional wet processings are necessary. Even when the former half of plate-making steps (image-forming processing) is simplified by the aforesaid digital processing, advantages due to the simplification are marginal if the latter half (development processing) is a complicated wet processing.
In particular, the whole industry has taken a great interest in the environment of the earth in recent years. From the viewpoint of consideration on the environment of the earth, too, the wet after-processing should preferably be simplified or changed to dry processing.
As one of the methods of omitting processing steps, there is a method called on-machine development wherein an exposed printing plate precursor is mounted on a cylinder of the printing machine and, while rotating the cylinder, a fountain solution and an ink are supplied thereto to thereby remove a non-image area of the printing plate precursor. That is, the method is a system wherein an exposed printing plate precursor is mounted as it is on a printing machine, and its processing is completed during an ordinary printing step.
The lithographic printing plate precursor suited for such an on-machine development has a photosensitive layer soluble in a fountain solution or an ink solvent and also has a light-room handleability suitable for being developed on a printing machine placed in a light room.
Conventional PS plates substantially fail to meet such requirements.
There has been proposed a lithographic printing plate precursor comprising a hydrophilic support having provided thereon a photosensitive layer wherein fine particles of a thermoplastic hydrophobic polymer is dispersed in a hydrophilic binder polymer (see, for example, Patent Literature 1). In making a printing plate, the lithographic printing plate precursor is exposed with an infrared laser to form an image by coalescing (fusing) the fine particles of the thermoplastic hydrophobic polymer by heat generated due to light-heat conversion, then the lithographic printing plate precursor having formed the image is mounted on a cylinder of a printing machine, and at least one of a fountain solution and an ink is fed to the precursor to conduct on-machine development. The lithographic printing plate precursor has a photosensitive region in an infrared region, and hence has a light-room handleability.
However, the image formed by coalescing (fusing) fine particles of a thermoplastic hydrophobic polymer has an insufficient strength, thus involving a problem with respect to printing durability of a printing plate.
A lithographic printing plate precursor containing microcapsules containing a polymerizable compound in place of the thermoplastic fine particles has been proposed (see, for example, Patent Literatures 2 to 7). The polymer image formed by the reaction of the polymerizable compound has better strength than that of the image formed by fusing fine particles.
Since the polymerizable compound has a high reactivity, there have been proposed many techniques for isolation thereof using microcapsule (see, for example, Patent Literatures 2 to 7). A thermally decomposable polymer is used as a shell of the microcapsule.
However, lithographic printing precursors described in Patent Literatures 2 to 7 above do not have sufficient developability on a printing machine.    Patent Literature 1: Japanese Patent No. 2938397    Patent Literature 2: JP-A-2000-211262 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)    Patent Literature 3: JP-A-2001-277740    Patent Literature 4: JP-A-2002-29162    Patent Literature 5: JP-A-2002-46361    Patent Literature 6: JP-A-2002-137562    Patent Literature 7: JP-A-2002-326470